Abstract Large sectors of the US population, especially certain racial and ethnic groups, remain underrepresented (UR) in the biomedical research (BMR) workforce. Although the National Institutes of Health has employed diversity- focused efforts to increase the proportion of UR scientists in BMR, these efforts have been largely focused at the undergraduate and predoctoral level. Racial/ethnic underrepresentation is evident at each stage of the BMR career trajectory, though the transitions from Postdoctoral Fellow (Postdoc) to Faculty and from Assistant to Associate Professor represent points of greatest risk for attrition from BMR. Inadequate mentoring, less access to role models, and isolation are implicated in racial/ethnic underrepresentation in academia. The dyadic mentoring model for UR groups is limited because few senior UR faculty exist and because it may not address the isolation experienced by UR scientists in BMR. Peer group mentoring is a strategy that builds a mentoring community and several facilitated (by a senior faculty) peer group mentoring models for UR Postdocs and Junior Faculty have shown promising outcomes. However, that research is uncontrolled and does not permit the disentanglement of the contributions of psychosocial support from those of skills-based mentoring. The primary objective of this proposal is to determine the unique contribution that psychosocial components of peer group mentoring make, above and beyond that of skills-based mentoring, to both personal gains and objective career outcomes for UR early career biomedical researchers using a facilitated peer group mentoring approach. The University of North Carolina and Duke University together will randomize 160 UR men and women Postdocs or Assistant Professors in BMR to one of two, 9-month, peer group mentoring arms, stratified by gender and rank, and facilitated by a senior UR BMR Faculty (with 6-8 peers/group),: 1) Skills-based only to facilitate manuscript and grant writing skills and other academic products; or 2) Skills-based + Psychosocial to include semi-structured discussions on topics such as microaggressions, the imposter syndrome, and cultural capital. Participants will be assessed for short-term (self-efficacy, belonging, vulnerability to stereotype threat, and professional identity), medium-term (career satisfaction and career commitment) and longer-term outcomes (NIH grant scores, funding, publications, citations, retention, promotion) at interim time points during and for a minimum of 2 years following the peer mentoring intervention. 50% of participants will provide qualitative data (interviews, focus groups, and written narratives) to elucidate the mechanisms by which the interventions influence outcomes. This research is expected to provide the evidence base for a scalable, effective approach to mentoring early career, UR biomedical researchers at other institutions.